The subject matter disclosed herein relates to spark gaps for use in ignition systems or other suitable systems.
Spark gaps are passive, two-terminal switches that are open when the voltage across the terminals is low, and then close when the voltage across the terminals exceeds a design value (e.g., 3 kV). The spark gap then re-opens when the current has fallen to a low level or when most of the energy from the voltage source is dissipated. Internally, the current is carried between two metal electrodes that are separated by a small ‘gap’ (˜mm) that is filled with a gas or gas mixture (e.g., Ar—H2—Kr) near atmospheric pressure. The gas is ordinarily insulating, but it becomes a conducting plasma ‘spark’ when the voltage between the two electrodes exceeds the design value which corresponds to the breakdown voltage.
For various applications, one parameter of interest may be the time between when a sufficient voltage is applied to the spark gap and the time at which it becomes conducting. This time corresponds to the ‘breakdown’ processes that initiate the transition of the gas from an insulator to a conductor.
There is an idealized but useful view of electrical breakdown as a two-step process—a ‘statistical’ time for the first electron to appear, followed by a ‘formative’ time for the electrons to ‘avalanche’ to a highly conductive state. A free electron appears at some time and location in the gap, and is accelerated by the electric field that is created by the potential difference between the electrodes. Once the electron gains sufficient energy there is some probability for it to ionize a gas atom or molecule and release a second free electron. Each electron is then accelerated and the process repeats, leading to an electron avalanche that makes the gas highly conducting. The energy gain and multiplication processes must overcome various energy and particle loss processes, and the first free electron should be created in preferred locations (e.g., at or near the negative electrode) for maximum effectiveness.
The time required for the second (avalanching) process is the ‘formative time lag’. It is generally short and can be practically ignored. Thus, the time required for the first process (the initial electron) is the ‘statistical time lag’, and it is this ‘first electron problem’ that is of primary interest in practice. In some devices such as laboratory apparatus or large electric discharge lamps the ‘first electron problem’ is solved by doing nothing more than waiting for a cosmic ray to create a free electron when it collides with a gas atom, gas molecule, or surface within the device. Electron-ion pairs are always being created at a given rate in atmospheric air by energetic cosmic rays that can easily penetrate into gas volumes within devices and structures. A Geiger counter is an example of a device that detects such events.
However, the ubiquitous cosmic-ray process cannot be relied upon to create effective free electrons within a required timeframe that may be needed for reliable operation of many devices that incorporate a spark gap. In particular, for device employing a spark gap the timeframe is typically too short to rely on a cosmic ray based process because the interaction volume (the region between the electrodes) is relatively small.
Instead, the conventional approach to solving the first-electron problem in a spark gap context (as well as in other devices dealing with similar issues, such as small electric discharge lamps) is to add a source of radioactivity, for example in the form of radioactive krypton-85 (e.g., 85Kr), which undergoes beta decay to emit an energetic (687 keV) electron, to generate seed electrons and reduce statistical time-lag to acceptable values. Other radioactive materials such as tritium or thorium are sometimes used. The addition of a radioactive component is sometimes referred to as ‘radioactive prompting’.
However, radioactive materials, even at trace level, are generally not desirable in a component or product because these materials add to of the cost of manufacturing, handling, and shipping.